Radio observations of a curious feature brought to the attention of astronomers by Dutch Galaxy Zoo volunteer Hanny van Arkel have finally revealed the nature of the object that came to be known as Hanny’s Voorwerp.

Like many ordinary Galaxy Zoo volunteers, Hanny was surfing through hundreds of galaxy images when she stumbled across a huge green irregular cloud of gas located about 60,000 light years from a nearby galaxy, IC 2497. Nicknamed Hanny’s Voorwerp (Dutch for object), its nature and origin have had astronomers scratching their heads for over a year.

The green gas cloud bears temperatures of over 15,000 degrees Celsius, but curiously, it is devoid of stars. Using the Westerbork Synthesis Radio Telescope (WSRT) and an e-VLBI array, an international team of astronomers led by Professor Mike Garrett of the Netherlands Institute for Radio Astronomy (ASTRON), and including Hanny van Arkel herself, have observed IC 2497 and the Voorwerp, to dig deeper into the mystery.

WSRT observations reveal a radio jet (white contours) emanating from the centre of the nearby galaxy IC 2497, headed straight in the direction of Hanny's Voorwerp (green). The observations also reveal a huge reservoir of hydrogen gas (coloured orange) that probably arose from a previous encounter between IC2497 and another galaxy. The presence of strong neutral hydrogen absorption (top right plot) argues that the central regions of IC2497 are highly obscured. Image: ASTRON/Dan Herbert/Isaac Newton Telescope.

The observations reveal a jet of highly energetic particles that are being generated by none other than a massive black hole lurking at the centre of the neighbouring galaxy. "It looks as though the jet emanating from the black hole clears a path through the dense interstellar medium of IC 2497 towards Hanny's Voorwerp", says Garrett. "This cleared channel permits the beam of intense optical and ultraviolet emission associated with the black hole to illuminate a small part of a large gas cloud that partially surrounds the galaxy. The optical and ultraviolet emission heats and ionises the gas cloud, thus creating the phenomena known as Hanny's Voorwerp.”

Another question that astronomers had was ‘where does all the hydrogen gas come from?’ The total mass of gas is about 5,000 million times the mass of the Sun and extends across hundreds of thousands of light years. Team member Dr Tom Oosterloo thinks that it has all the hallmarks of an interacting system. “The gas probably arises from a tidal interaction between IC 2497 and another galaxy, several hundred million years ago,” he says. "The stream of gas ends three hundred thousand light years westwards of IC2497 - all the evidence points towards a group of galaxies at the tip of the stream being responsible for this freak cosmic accident".

Hanny van Arkel shares the excitement of the professional team of astronomers, and visited ASTRON to find out more about the object she stumbled upon. "I'm happy we are making progress,” she says. “Apparently the more we learn about the Voorwerp, the more intriguing it becomes".

The team still think that the Voorwerp has a few more secrets to reveal, and plan much deeper observations with the WSRT and other higher resolution radio telescopes soon.

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